In an international context characterized by a fast growth in fuel requirements, in particular gas oil bases in the European community, the search for new renewable energy sources that can be integrated in the conventional refining and fuel production scheme is a major challenge.
In this respect, integration in the refining process of new products of vegetable or animal origin, resulting from the conversion of lignocellulosic biomass or from the production of vegetable oils or animal fat, has known renewed interest as a result of the increase in the cost of fossil materials. Similarly, conventional biofuels (ethanol or vegetable oil methyl esters mainly) have acquired the status of complement to petroleum type fuels in gasoline pools. Furthermore, the processes known to date that use vegetable oils or animal fat are the cause of significant CO2 emissions, known for their negative effects on the environment. A better use of these bioresources, such as their integration in the gasoline pool, would therefore be of great advantage.
The production of fuel bases is more and more identified as a new attractive outlet by the agricultural world, in particular by vegetable oil producers, these oils resulting from the crushing of oilseed such as rape, soybean or sunflower. In fact, these vegetable oils consist of fatty acids in form of triglycerides, with long alkyl chains whose structure corresponds to the normal paraffins of gas oil cuts (chain length from 12 to 24 carbon atoms, depending on the nature of the vegetable oil). Unsuitable for directly feeding modern diesel engines, these vegetable oils first have to be converted.
One existing approach is based on the reaction of transesterification with an alcohol such as methanol, leading to vegetable oil methyl esters (VOME) commonly referred to as biodiesel. This option is now widely used in Europe since the production of VOME has increased very significantly during the past ten years, thus reaching 1.5 Mt in 2003 (the average yearly growth rate is 35% between 1992 and 2003). This production is notably supported by the European directive on the promotion of biofuels (2003/30/CE) that sets increasing biofuel consumption objectives in the field of transport. These consumptions will have to be at the minimum 2% in 2005, 5.75% in 2010 and 8% (percentages measured in energy) in 2015 of the overall consumption of gasoline and gas oil used for transport. However, this type of process is relatively expensive and it requires vegetable oil type limitations in order to meet the biodiesel specifications. Besides, the feeds for this type of process must be carefully selected, so that a certain number of vegetable oils cannot be treated in this manner. Finally, the cold properties of these products also represent a limiting factor.
Another approach consists in directly using vegetable oils via their conversion to elementary fatty acid derivatives, by means of hydrorefining or hydroconversion processes whose catalysts are also known to the person skilled in the art for their hydrodeoxygenation properties [E. Laurent, B. Delmon, Appl. Catal. A 109 (1) 1994 77-96 and 97-115]. In this case, the triglycerides are converted to mainly paraffinic and saturated derivatives, thus forming excellent bases for the gas oil pool considering their good cetane numbers.
There is therefore a strong need of the petroleum refining industry to treat oils of vegetable or animal origin, if possible at a lower cost and while taking account of the existing plants.
Several patents cover these fields of interest.
U.S. Pat. No. 5,233,109 describes the implementation of thermal or catalytic cracking of vegetable oils leading to a wide range of products such as paraffins, but also aromatic derivatives and unsaturated derivatives in the boiling range of gasolines and gas oils. This method produces derivatives that cannot be directly used as gas oil fuel bases and it is particularly penalizing as regards meeting standard specifications (oxidation stability).
U.S. Pat. Nos. 4,992,605 and 5,705,722 describe methods of producing bases for the gas oil pool produced from direct conversion of vegetable oils (rape, palm, soybean, sunflower) or of lignocellulosic biomass to saturated hydrocarbons after hydrotreatment or hydrorefining of these products alone.
The conversion methods described are operated at pressures ranging between 0.48 and 1.52 MPa and at temperatures ranging between 350° C. and 450° C. allowing to obtain products with a high cetane number. The pro-cetane additives thus obtained are mixed with the gas oil in proportions of 5 to 30% by volume.
Furthermore, these two patents have the major drawback of a high hydrogen consumption essentially due to the unsaturations present in the feeds consisting of vegetable oils and to the fact that the oxygen contained in the triglycerides is generally decomposed by hydrodeoxygenation in the presence of a hydrotreating catalyst.
In this respect, patent EP-1,681,337 represents an improvement since it provides a method using only small amounts of hydrogen. It is a decarboxylation/decarbonylation method on group VIII catalysts first reduced to a temperature ranging between 100° C. and 500° C. The reactions are then carried out at a temperature ranging between 200° C. and 400° C. at a pressure ranging between 1 and 15 MPa. In this case, the decarboxylation/decarbonylation reaction produces saturated hydrocarbons and CO2 or CO respectively. Hydrogen is no longer necessary, except for keeping the catalytic phase in metallic form and/or for preserving the catalyst from too fast a deactivation.
Besides, operating at such temperatures increases the risks of polymerization of the vegetable or animal feeds containing many unsaturations, which leads to the formation of solid particles causing operability problems for fixed-bed plants.
The products formed by means of this method are not directly used as fuel bases because of very poor cold properties.
In order to overcome these limitations on the cold properties, other patents describe a sequence of a stage of hydrogenation and isomerization of vegetable oils so as to obtain a mixture of branched saturated hydrocarbons whose cold properties are known to be higher than those of the same compounds, but non-branched. Patent FI-100,248 relates to the hydrogenation of fatty acids or triglycerides to n-paraffins, followed by an isomerization stage. Patent EP-1,396,531 describes a hydrotreating method for carrying out hydrodeoxygenation under a pressure of 5 to 10 MPa and at a temperature of 300° C. to 500° C., followed by an isomerization stage at a pressure of 2 to 10 MPa and a temperature of 300° C. to 400° C. The operating temperatures are critical as regards the operability risks linked with the degradation of vegetable oils.
Finally, patent EP-1,693,432 describes a method allowing hydroconversion of a mixture of vegetable oils (1% to 75% by volume) and of hydrocarbons (99% to 25% by volume) in a single hydrotreating reactor, at a pressure of 4 MPa to 10 MPa, with a NiMo or CoMo type catalytic bed operated at a temperature ranging between 320° C. and 400° C. The advantage of this approach is the gain in terms of cetane number and of density decrease provided by mixing with the vegetable oil in relation to the properties obtained by direct treatment of the petroleum base. Furthermore, mixing hydrocarbon feeds with vegetable oils allows to improve the cold properties of the effluents obtained in comparison with those that would be obtained by treating the vegetable oils alone. The hydrotreating catalysts used are group VIB transition metal sulfides promoted by group VIII metals. The presence of compounds from non-desulfurized petroleum cuts allows to obtain a higher H2S partial pressure than the minimum partial pressure necessary for the catalyst stability.
This method however involves several drawbacks. The first drawback lies in the implementation of a single stage for co-treating the vegetable oil and the petroleum base. In fact, this is limiting as regards optimum operation of hydrotreating catalysts intended to operate the decarboxylation-decarbonylation, hydrodeoxygenation and hydrodesulfurization reactions simultaneously. Now, the operating conditions allowing to promote the decarboxylation and decarbonylation reactions, and thus to obtain a hydrogen consumption reduction in relation to hydrodeoxygenation are milder than those required to obtain the desired effluent sulfur specifications. The activity and the stability of the catalyst as used in this patent are penalized because of the formation of the coproducts of the hydrodeoxygenation and decarboxylation and/or decarbonylation reactions, i.e. water and CO and/or CO2. These molecules are in fact well known to the person skilled in the art for their deactivation and inhibition effects respectively on hydrotreating catalysts (US-2003/0,221,994). It would therefore be interesting to be able to do without these coproducts so as to allow a better catalyst activity while limiting the presence of inhibitors and to obtain a longer cycle time of the plant (catalyst stability while limiting the harmful effects due to the presence of water). Co-treatment of the petroleum base and of the vegetable oil thus leads to fast catalyst aging and to a degradation of the hydrodesulfurization performances of the catalysts. In particular, this patent does not mention hydrodesulfurization performances of the process or the quality of the products formed in relation to all the standard specifications for gas oil fuels. Considering the cost of the loading and unloading operations, as well as the cost of the raw material for the catalysts and recycling thereof, it is important for refiners to maximize the cycle time of the plant and consequently the life of the hydrotreating catalyst allowing to obtain gas oils meeting specifications.
Finally, the orientation of the vegetable oil conversion mechanism (hydrodeoxygenation or decarboxylation/decarbonylation) is difficult under the operating conditions required to carry out hydrodesulfurization of the gas oil base. Now, this selection is very important in terms of hydrogen consumption. In fact, it is not unknown to the person skilled in the art that the hydrogen consumption remains a critical parameter, considering its low availability in refineries. It is therefore important to minimize its consumption. The fact that the optimum operating conditions differ for the desired reactions is thus greatly limiting in the case of co-treatment of vegetable and petroleum oils carried out in a single stage.
The second drawback of this method lies in the heating procedure for the vegetable oil prior to feeding it into the mixer, then into the hydrotreating plant proper. In fact, it is known to the person skilled in the art that the temperature rise (>150° C.) of vegetable oils greatly favours the formation of gum or of heavy polymers through thermal degradation or thermo-oxidation of a vegetable oil (A. Rossignol-Castera. “La thermo-oxydation des huiles végétales” Institut des corps gras ITERG-2006). This phenomenon is intensified by the presence of unsaturations of the fatty acids and of traces of metals (such as Cu, Fe, Zn, Al). These reactions mainly produce polymers of triglycerides or previously oxidized triglycerides, either by formation of epoxide bridges or by oligomerization of the double bonds (radical mechanism) (J. L. PERRIN et coll. “Etude analytique profonde d'huiles chauffées—Techniques analytiques et essais préliminaires” Revue francaise des corps gras, 1992, vol. 32, No4, p. 151-158). These compounds may hinder the progress of the process because they are likely to clog the reactor or to generate unwanted degradation products.
Besides, other compounds present in minor proportions in the vegetable oils (case of phospholipides, proportion typically below 1%) are likely to generate under temperature gels inducing pressure drops during the course of the process. The fact that the heating and operating conditions described in this patent are severe can lead to vegetable oil degradations harmful to the smooth running of the plant (clogging, pressure drop, operability).
There is therefore an industrial and environmental need to improve the co-treatment conditions of mixtures of oils of vegetable or animal origin and of petroleum bases in order to produce gas oil fuels.
The use of two catalyst beds with intermediate separation is well known to the person skilled in the art. When all of the effluent from the first bed is sent after stripping to the second one, an integrated process can be used as described in document U.S. Pat. No. 5,110,444. When a part only of the effluent of the first bed is sent to the second, another type of integrated process has been described in patent application WO-2003/044,131A1. In these processes, the second catalyst bed is used as a finishing bed for deepening the conversion of the sulfur, nitrogen or aromatic compounds performed in the first bed. Thus, addition in admixture with the effluents of the first catalytic section, which furthermore generates quenching, of a stream external to the process requiring in the second bed a new type of catalytic reaction that does not occur in the first catalyst bed is not described in these documents.